This invention relates to a closure or cover plate for an opening and more particularly relates to a cover plate such as a "Butterworth cover" for sealing an opening through the deck of a cargo vessel.
In a typical marine vessel (e.g., an oil tanker) used for transporting fluid products, a plurality of large cargo tanks are positioned in the hull immediately below the deck. At least one relatively large opening if formed through the deck into each tank to provide a main entry into that tank. As is understood in the art, normally several additional, smaller openings are provided through the deck into each tank through which various service operations on the tanks and/or cargo can be carried out. When the service openings are not in use, they are normally sealed by removable cover plates, commonly referred to as "Butterworth covers".
Originally, a Butterworth cover was secured over a service opening by forming holes in the cover which fit over studs fixed to the deck around the service opening and then threading nuts onto the studs. Besides requiring a substantial amount of time and effort to remove and replace a cover plate, the studs had a tendency to corrode and often required replacement. To alleviate such problems, quick-release cover plates were developed.
One such cover plate is disclosed in U.S. Pat. No. 3,184,099, issued May 18, 1965. This cover plate incorporates a cammed latching means which secures or releases the cover plate to or from a deck opening with a simple turn of the latch. However, as recognized in U.S. Pat. No. 3,184,099, the thickness of the deck to which the cover plate is to be secured may vary from cargo vessel to cargo vessel. In order for a cover plate to find widespread utility, it must include some adjustable means to compensate for the various deck thicknesses. The cover plate of U.S. Pat. No. 3,184,099 uses different sizes of shim washers on the shaft of the latch means for this purpose but the latch means has to be disassembled to rearrange the washers to adapt the cover plate for use with a particular deck thickness.